Optically finished glass lenses press molded from alkali metal fluoroaluminophosphate compositions as disclosed in U.S. Pat. No. 4,362,819 (Olszewski et al.) have been marketed commercially by Corning Incorporated, Corning, N.Y. The molds utilized in that forming process have been prepared from various commercially available lead silicate glasses The use of the glasses in the fabrication of molds was based primarily upon two factors, viz., their viscosity/temperature characteristics and their thermal expansions. With respect to the first factor, the glass to be used as the mold must itself not only be moldable at a relatively low temperature, typically within the range of about 450.degree.-560.degree. C., but also it must be rigid at the temperature at which the lens blanks are pressed, generally about 350.degree.-400.degree. C. On the basis of known viscosity/temperature slopes, that requirement translates into a range of glasses having softening points between about 500.degree.-620.degree. C. With regard to the second factor, the linear coefficient of thermal expansion of the mold glass over the temperature interval 25.degree.-300.degree. C. must lie between about 46-142.times.10.sup.-7 /.degree.C. in order to be compatible with the various metals employed in the pressing apparatus. Furthermore, experience has indicated that: first, the linear coefficient of thermal expansion of the mold glass ought to be considerably lower than that of the lens glass (generally about 150 .times.10.sup.-7 /.degree.C. over the temperature interval 25.degree.-300.degree. C.) so that the molded glass lens will tend to release from the mold upon cooling from the pressing temperature; and, second, that the linear coefficient of thermal expansion of the mold glass ought to be substantially greater than that of the molding sleeve (commonly prepared from tungsten carbide which has a linear coefficient of thermal expansion of 46.times.10.sup.-7 /.degree.C. over the temperature interval 25.degree.-300.degree. C.) so that the finished mold is readily extractable from the molding assembly.
The lead silicate glasses currently employed for making the molds have five significant drawbacks:
First, because their bulk compositions differ greatly from that of the lens glass, the large chemical gradients that exist at the interface between the mold and the lens during the pressing process favor mass transfer between the two glasses;
Second, it is believed that, because it is a silicate, the mold glass is especially susceptible to attack by the fluorine of the lens glass;
Third, it appears that the lead in the mold glass is reduced to metallic lead during the pressing process inasmuch as microscopic-sized pits are developed in the mold surface which seem to be centered about particles of elemental lead, those imperfections having adverse effects on both the mold lifetime and the lifetime of the metal master from which the mold itself is pressed;
Fourth, the viscosity curves of the current mold glasses are relatively shallow which is manifested by the quite large temperature interval between the annealing and softening points thereof, viz., about 165.degree.-185.degree. C.; and
Fifth, the linear coefficients of thermal expansion of the mold glasses are relatively high, resulting in a less than optimum mismatch in coefficient of thermal expansion between the lens glass and mold glass.
Therefore, the primary objective of the present invention was to devise glass compositions which, by avoiding the above five drawbacks, would be eminently suitable for the forming of glass molds for use in pressing optically finished glass lenses.
A specific objective of the instant invention was to design glass compositions especially suitable in the construction of glass molds for use in contact with alkali metal fluoroaluminophosphate glass compositions of the type disclosed in U.S. Pat. No. 4,362,819, supra, and alkali metal, alkaline earth metal, zinc phosphate glass compositions of the type disclosed in Ser. No. 599,751 above.